This invention relates to gate valve structures and more particularly to gate valves having an expanding gate assembly formed of two portions which expand outwardly from each other at open and closed positions to provide a fluid-tight seal with the adjacent face seals.
Normally floating seat assemblies are mounted within recesses of the valve body on opposite sides of the gate structure and fluid under pressure may enter recesses behind the seat assemblies to cause the seat assemblies to move or float toward the gate assembly. In the event the upstream seat assembly is subjected to a high differential pressure when the fluid pressure in the adjacent flow passage is substantially higher than the fluid pressure in the valve body, then the upstream seat assembly will "float" or move toward the gate assembly especially in the closed position of the gate assembly thereby to cause sticking or binding of the gate assembly when the gate assembly is initially moved from the fully closed position toward the open position.
As shown in U.S. Pat. No. 3,823,911 dated July 16, 1974, spacers are provided between floating seats in an expanding gate valve structure to limit the movement of the seats toward the gate valve assembly. This arrangement provides a minimum clearance between the seats at all times and has been effective to minimize the seat assemblies from binding against the gate assembly in a fully closed, collapsed position such as might occur when the upstream seat is subjected to a relatively high differential pressure.